1. Field of the Invention
The present invention relates to the field of optical lithography and, more particularly, to a specialized lens arrangement for improving imaging performance of lithography tools.
2. Background of the Related Art
The use of optical lithography to pattern devices that are fabricated on a semiconductor wafer (such as a silicon wafer) is well known and has been in practice for many years. Generally, a mask is used in which light absorbing elements present in the mask define a positive or a negative pattern. One common practice is to position light absorbing chrome on a transparent mask material, such as glass (or quartz). The light not absorbed by the chrome is projected through the mask and exposes a photosensitive material to form a latent image therein. Depending on the positive or negative nature of the photomask technique and the photosensitive layer (photoresist) employed, the exposed (or the un-exposed) area is removed. The removed photoresist exposes underlying areas which are then processed by various processes, including etching.
In a typical microlithography tool, a light source is used to illuminate the mask (reticle) so that a pattern present on the mask is projected onto the photoresist to form a latent image in the photoresist. It is preferred to have uniform illumination on the mask by the light source, so that light intensity across the reticle plane is also uniform in intensity. It is appreciated that non-uniform intensity at the reticle plane can result in non-uniform exposure dose of the photoresist. A significant variation in the exposure dose of the photoresist present on a wafer can result in non-uniform feature formation when the integrated circuit is fabricated. For example, non-uniform exposure dose on photoresist can result in sizeable variation in the linewidth of the pattern being imaged.
With ordinary light sources, the light emitted by the source is usually of low optical coherence so that a fairly uniform light intensity profile is achieved at the reticle plane by superposing multiple wavefronts. However, with coherent light sources, such as a laser, the light projection is confined to a collimated beam that is of relatively high coherence. Thus, light intensity across the reticle field is not necessarily uniform as a result of the coherence properties of laser light. In order to obtain a more uniform intensity profile at the reticle plane, some projection systems use a light dispersion mechanism to disperse spatially the light beam from a coherent light source. One such mechanism is a lens arrangement known as a fly's eye array (or fly-eye). A fly's-eye illumination system utilizes a number of lenses arranged as a two dimensional array to disperse the beam into a number of "beamlets" for projection of light onto a target plane. The structure and use of fly-eye lens arrangements are known in the art. See for example "Illumination System of an Excimer Laser Stepper" by Yutaka Ichihara et al., SPIE Vol. 1138 (1989) at p.137.
Although uniform illumination of laser beams can be sought with the use of fly's-eye lens arrangement, a negative effect is noted with the use of coherent light sources. This effect is the spatial coherence associated with narrow band light sources, such as lasers, and the manifestation that is known as speckle. The high degree of spatial coherence of line narrow lasers produces an unwanted interference pattern (speckle) superimposed over ideally uniform light intensity illumination at the reticle plane. The interference effect of spatially coherent light beams producing speckle is known in the art. See for example, "The Coherence Factors of Excimer Laser Radiation in Projection Lithography" by K. A. Valiev et al., J. Vac. Sci. Technology, B7 (60), Dec/Nov 1989 at p.1616.
The present invention describes a scheme in which a fly-eye lens arrangement is used for obtaining uniform light intensity illumination of a narrow band coherent light source, but in which speckle is reduced or removed.